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<br /> 100 YR. FLOOD HYDROGRAPH <br /> MONTROSE ARROYO <br /> CONFLUENCE WITH CEDER CREEK <br /> 1200 <br /> " 110 CF S <br />1000 <br /> UPSTREAM <br /> BASlfl <br /> 000 RUNOFF <br /> . <br /> I <br /> I I <br />_600 \1 <br />" I <br />" <br />u ~tCFS <br />- <br />u I <br />0 <br />" "" <br />. , , , <br />. , , <br />u I UlC" \ I <br />" <br />0400 \. ~I <br /> RUNOFF <br /> ~ <br /> I I <br /> 200 , . <br /> I , <br /> 0 "" <br /> 0 SO "0 200 250 <br /> TIME(MINUTES l <br /> FIGURE 5 <br /> <br />HYDRAULIC ANALYSIS <br /> <br />A hydraulic analysis WdS completed by Hydro-Triad Ltd. inAprll,1979. <br />(Reference 1) to determine ~ater surface profiles for the 10-year, 50-year, <br />lOO-yeu, and 500-year floods on Montrose P.rroyo, Cedar Creek and Dry Cedar <br /> <br />Creek. These water surface elevations were computed using theHEC-IIco m- <br /> <br />putermodel for backwater analysis. The culvert crossings on Montr<lse <br />Arroyo ~ ra insuffi dent to pa 55 the 1 DO-year flow; consequentl y. d'jri ng <br />such an event a considerable amount of flow would be lost from the channel <br /> <br />" <br />;! <br /> <br />to flow overland. Thisshal1owfloodingwouldflowawayfY'Onli"ontrose <br /> <br />Arroyo west and northwest to the Uncompahgre River. Some of this flow <br /> <br />'Houldpond along the east side of theO&RGW Railroad embankrrent betw een <br />South 1 st and North 2nd Street~. The previous h"vdraul ic computations were <br />carried out based on the premise that inrnediately downstream of each cross- <br /> <br />ingall spillcdflowisconsidcredtobebackinthechannel,andanyex- <br />cess is again shown as spi11ingat the next inadequate crossing. This <br />procedure WdS used by Hydro-Triad Ltd. to prevent the analysis honl be~olll- <br /> <br />;ng ~urprsf'eded each time an UPS tream cul vert is improved, and to ill ustrate <br /> <br />a conservative situation. <br /> <br />In order to evaluate existing damages, it is Tlecessary to have the <br />floodplain andflocd profiles defined for the existing conditi ons. Therefore, <br />Gi~gory Associates conducted hydraulic analyses. to determine flood elevations <br /> <br />a~d del ineate floodi,,'J limit~ th~t rpflf'(:t thp actL",l s ituatiQn that exists <br /> <br />in :-Iontrose. The analyses were performed in the following manner: ~rocefd~ <br />inginadownstreamdirect;on, at the first inadequate crossing ( South 12th <br />Street). cal ~ulations were made based on culvert rating CJrves and existing <br />topography to determine the amo~nt of flow lQs~ fro!" the channel. Downstream <br />Of the crossing tile discharge in the Channel '''~, reduced by this dill")Unt. Thh <br /> <br />-19- <br />